Thermoplastic film production

ABSTRACT

In a process for extruding thermoplastic film means are provided for maintaining substantially free from deposit a wire electrode which is used to deposit an electrostatic charge on to the upper surface of the film. In a preferred embodiment the wire electrode is continuously unwound from a first reel on to a second reel. The wire electrode is kept substantially free of deposit by means of either a scraper plate or a current of air which is passed between the film and the wire electrode to prevent deposits from forming.

This invention relates to thermoplastic film production and improvementstherein.

Such film may be produced by the extrusion of polymer through a die ontoa casting drum. It is essential that there should be adequate contactbetween the freshly extruded film and the casting drum in order toensure good heat transfer.

A known method of ensuring such contact is by depositing anelectrostatic charge on the upper surface of the extruded film whilemaintaining the casting drum at a low potential, e.g., by allowing thedrum to be earthed. The polymer composition which is extruded from thedie often contains additives, e.g., heat or U.V. stabilizers orantistatic agents and these may be volatile at the temperature ofextrusion.

When an electrostatic pinning system which normally uses a wireelectrode is used in conjunction with such a composition containingvolatile additives, it is found that degradation products of theseadditives are deposited on the electrode and this not only raises thethreshold ionization potential for the electrode but also markedlyalters the appearance of the discharge between the wire and the film.The consequences are that the efficiency of pinning is reduced leadingto an irregular of patchy adherence of the film to the casting drum anda general and "noisy" blue glow fills the space between the wire and thefilm instead of there being present only a faint "silent" glow in theimmediate vicinity of the wire itself.

It is an object of the present invention to overcome the abovedifficulty.

Accordingly we provide a process for quenching a freshly extrudedthermoplastic composition which comprises extruding a moltenthermoplastic film forming polymeric composition on to a moving castingsurface and depositing on the upper surface of the film an electrostaticcharge from an electrode situated near to said film surface andmaintained at a large electrostatic potential difference from thecasting surface so that the film is caused to adhere to said castingsurface, in which means are applied to maintain said electrodesubstantially free from deposit arising from condensed vapours ofvolatile matter present in said film forming composition or thedecomposition products of said volatile matter. By "substantially freefrom deposit" we mean that the electrode, since it does not becomecontaminated by deposit, continues to function successfully for aconsiderable time, e.g., eight hours' running time, i.e., a timeconsiderably longer than that required to set the machine operatinguniformly, without there being any need to adjust the potentialdifference between the wire and the cast film to maintain satisfactoryperformance. Signs of a deterioration in performance show up in theappearance of the cast film, e.g., short transverse streaks consistingof air bubbles may appear on the film. If the film is subsequently drawnthese bubbles may initiate breaks in the film. A further indication ofunsatifactory working is the appearance on the electrode wire of avisible dark-colored deposit. In the successful working of the presentinvention no such deposit occurs, although a very thin uniformstraw-colored deposit which does not significantly affect the efficiencyof pinning may be formed.

It is preferred to use a wire electrode and this may be maintainedsubstantially free from deposit by the means of a cleaning device, e.g.,a scraper plate having an orifice of sufficient diameter just to allowpassage of the clean wire, or a bath containing a solvent for thedeposit, may be periodically passed along the wire.

Alternatively, the wire may be reciprocated through such a cleaningdevice or the wire may be in the form of an endless band which may becontinuously passed through such a cleaning device.

In another and preferred alternative, a clean wire is continuouslyunwound from one reel at one side of the die orifice and wound onto asecond reel at the other side. Since a suitable speed of transversemovement of the wire which prevents the dwell time of the wire above theextrudate being sufficient to accumulate substantially any deposit, isabout 1 ft./hour, such a system will run for very many hours beforereplacement with a fresh coil of clean wire is required and thereplacement or cleaning of the wire ceases to be a limiting factor inthe extended running of the equipment.

Such devices will, of course, be at the same potential as the electrodewire and will therefore require to be insulated.

Another method of maintaining the electrode substantially free fromdeposit comprises preventing the vapour arising from the volatileadditives from reaching the electrode or reducing the concentration ofsuch vapour in the vicinity of the electrode to negligible proportions.

This may conveniently be effected by passing a current of air betweenthe film and the electrode. Since such a current of air will normallyimpinge on the die it is desirable that it should be at approximatelythe same temperature as the die, that is the temperature of the extrudedmaterial, since otherwise there would be set up a temperaturedifferential between different parts of the die which could result invariations in the profile of the extruded film.

Any conventional system for producing a current of hot air may be used,e.g., a fan blowing air over a heated wire or grille.

Alternatively, a suction device may be mounted near to the die to removethe volatiles.

Any thermoplastic film may be used in the present invention for examplefilms of polymers and copolymers of polyethylene, polypropylene andother polyolefines, polyethylene terephthalate, polyamides, polyvinylchloride, polyoxymethylene or poly-3,3-.[.biscolhromethyl.]..Iadd.bischloromethyl .Iaddend. oxacyclobutane.

Examples of volatile additives are antistatic agents and antistaticpromoting agents, e.g., fatty acid amides such as oleamide or erucimideand quaternary ammonium and phosphonium salts which contain a long chainof carbon atoms.

Examples of U.V. stabilizers are benzophenones, salicylates andbenzotriazoles and examples of heat stabilizers are dialkylthiodiacylates and condensates of a phenol with an aldehyde or a ketone.

The films produced according to this invention may, if orientable,subsequently be drawn in one or more directions. They may be also slitinto narrow tapes before or after drawing preferably before drawing asdescribed in our copending British patent application No. 1,424/66.

The invention will now be described with reference to the accompanyingdrawings in which

FIG. 1 is a diagrammatic sectional elevation showing the invention and

FIG. 2 shows an alternative electrode system.

In FIG. 1, polypropylene film 1 containing 0.5% of a volatile U.V.stabilizer is extruded from die 2 on the chilled quenching drum 3.Quenching drum 3 is connected to earth as shown at 4 and wire 5 which isconnected to a source of high potential forms the electrode. Hot airblower 6 which is made up of a centrifugal fan and an electric heatingelement directs a stream of air at a temperature of from 200°-250° C.between the electrode and the quenching film.

In FIG. 2, the electrode is formed by wire 7 which is in the form of anendless band driven by a rotating pulley 8 through orifices 9 in scraperplates 10. The invention is illustrated but in no way limited by thefollowing examples. In the examples all percentages are by weight.

EXAMPLE 1

Polypropylene containing 0.5% 2-hydroxy-4-methoxybenzophenone asultraviolet light stabilizer was extruded and cast as film 0.006 inchthick on a steel drum maintained at 75° C. and running at acircumferential speed or 45 ft./minute. The width of the cast film was 9inches and the hourly mass of film passing between wire and drum in thearrangement indicated schematically in FIG. 1 was about 60 lb./hour. Theamounts of U.V. stabilizer found by analysis before extrusion andafterwards were respectively 0.47% and 0.27%. The electrode comprised anickel chrome wire of 0.005 inch diameter and the initial potentialdifference between the electrode and the wire was 4.5K volts, the totalcurrent being 150 μamps. The velocity of the air current produced by theblower was initially adjusted by choking down the air intake of theblower so that the upper surface of the molten film should not beroughened by the impact of turbulent air; the temperature was adjustedby variation of the current supplied to the heating elements to 280° C.,i.e., the die temperature. Alternate periods of 8 hours were occupied byrunning with the blower on and then off. It was found that with theblower on trouble-free running was possible for at least 8 hours, i.e.,no adjustment of the position of the wire nor alternation of thepotential difference was necessary. At the end of the period of runningwith the blower on the uniform deposit on the wire was very slight,being yellowish brown in color. When the blower was switched off thewire begain to deteriorate rapidly; after an hour it was visiblydarkening and becoming patchy and after 3 to 4 hours it became necessaryto remove and clean or replace the wire because the quality of cast filmwas no longer acceptable.

EXAMPLE 2

The same extrusion conditions of rate and temperature were used as inthe case of Example 1. A nickel chrome wire of the same diameter and atthe same potential as in Example 1 was used and this was mounted in sucha way that it could be unwound from a first reel at one side of theextruding film and wound onto a second reel at the other side by amanually operable electrically insulated handle attached to the secondreel. This was wound at 15 minute intervals by an operator, about 3inches of wire being wound on each occasion.

Using the equipment it was possible to obtain extended trouble-freerunning.

EXAMPLE 3

In this example the same polymer composition was used as in the previousexamples. The width of the cast film was 18 inches and the extrusionrate was 150 lb./hour. The electrode was a Nichrome wire of diameter0.009 inch which, as in Example 2 was fed from a first reel at one sideof the extruding film to a second reel at the other side. The takeoffreel was electrically driven so that the rate of travel of the wire was18 inches per hour.

Extended trouble-free running was attained.

We claim:
 1. A process for quenching a freshly extruded thermoplasticcomposition which comprises extruding a molten thermoplastic filmforming polymeric composition on to a moving casting surface, depositingon the upper surface of the film an electrostatic charge from a wireelectrode situated near to said film surface and maintained at a largeelectrostatic potential difference from the casting surface so that thefilm is caused to adhere to said casting surface, and maintaining saidwire electrode substantially free from deposit arising from condensedvapors of volatile matter present in said film forming composition orthe decomposition products of said volatile matter by continuouslyunwinding said wire electrode from a first reel positioned at one sideof the die orifice from which the thermoplastic film is extruded,winding said wire on to a second reel positioned at the other side ofthe die orifice at a speed which prevents the dwell time of the wireelectrode above the extruded film from being sufficient forsubstantially any deposit to accumulate on the said wire.
 2. A processaccording to claim 1 in which said wire electrode is maintainedsubstantially free from deposit by the means of a scraper plate havingan orifice of sufficient diameter just to allow passage of the cleanwire.
 3. A process according to claim 2 in which said electrode wire isreciprocated through said scraper plate.
 4. A process according to claim2 in which said electrode wire is in the form of an endless band whichis continuously passed through said scraper plate.
 5. A processaccording to claim 1 in which said wire is maintained substantially freefrom deposit by means of a bath containing a solvent for the depositwhich is periodically passed along the wire.
 6. A process according toclaim 5 in which said electrode wire is reciprocated through said bath.7. A process according to claim 5 in which said electrode wire is in theform of an endless band which is continuously passed through said bath.8. In a process for quenching a freshly extruded thermoplasticcomposition which comprises extruding a molten thermoplastic filmforming polymeric composition on to a moving casting surface, depositingon the upper surface of the film an electrostatic charge from a wireelectrode situated near to said film surface and maintained at a largeelectrostatic potential difference from the casting surface so that thefilm is caused to adhere to said casting surface the improvementconsisting of maintaining said wire electrode substantially free fromdeposit arising from condensed vapors of volatile matter present in saidfilm forming composition or the decomposition products of said volatilematter by passing a current of air between the upper surface of saidthermoplastic film and the wire electrode so that said volatile matteris substantially prevented from reaching the electrode, said current ofair being maintained at approximately the same temperature as the dieorifice through which the thermoplastic film is extruded. .Iadd.
 9. In aprocess for quenching a freshly extruded thermoplastic composition whichcomprises extruding a molten thermoplastic film forming polymericcomposition onto a moving casting surface and depositing on the uppersurface of the film an electrostatic charge from an electrode situatednear to said film surface and maintained at a large electrostaticpotential difference from the casting surface so that the film is causedto adhere to said casting surface, the improvement which comprisespreventing condensible vapors issuing from the molten film fromcondensing on said electrode by maintaining a current of heated airbetween the electrode and the film, said air being heated by meansadditional to any heat from the film. .Iaddend. .Iadd.
 10. Apparatus forpreparing film from an extrudible thermoplastic composition whichcomprises extrusion means including a die orifice for extruding a filmof said thermoplastic composition which issues condensible vapors; amoving casting surface; electrode means near said surface but spacedtherefrom; means for maintaining said electrode means at a largeelectrostatic potential difference from said casting surface so thatextruded film is caused to adhere to said surface, and means formaintaining the electrode means substantially free from deposit ofcondensed vapors issuing from the extruded film..Iaddend.